When a link disconnection occurs in a network device such as a network relay device or a media converter, a technique of notifying the counterpart device of occurrence of a link disconnection and forcibly disconnecting a link between the counterpart device and the subsequent device is known (For example, see Patent Document 1). Such control is called link loss forwarding. The grounds for performing link loss forwarding are to allow the subsequent device to recognize occurrence of a link disconnection so as to stop useless data transfer which will be wasted or to transfer data via a detour path.
When performing link loss forwarding, a deadlock may occur. As shown in FIG. 12 for example, when a link disconnection “a” occurs between a network relay device 100 and a L2SW (Layer 2 Switch) 101, the network relay device 100 notifies a network relay device 200, which is a counterpart device, of occurrence of the link disconnection “a”. Upon receiving the notification, the network relay device 200 forcibly causes a link disconnection “b” between it and the L2SW 201. Further, the network relay device 200 notifies the network relay device 100, which is the counterpart device, of occurrence of the link disconnection “b”. With this notification, as the network relay device 100 recognizes that the link disconnection “b” occurred in the counterpart device, even when the link disconnection “a”, which is the original failure, is recovered, the network relay device 100 forcibly causes the link disconnection “a”. As such, the network relay devices 100 and 200 are in a deadlock state where the respective devices wait for link recovery of the counterpart device.
In order to prevent occurrence of such a deadlock state, Patent Document 1 discloses that after the link disconnection “b” is forcibly caused by the network relay device 200, a link disconnection transfer starting frame for instructing prohibition of a forcible link disconnection between the network relay device 100, which is the counterpart device, and the L2SW 101 is transmitted to the network relay device 100. Thereby, when the link disconnection “a” is recovered, the network relay device 100 never causes a forcible link disconnection, so that occurrence of a deadlock state can be prevented.    Patent Document 1: JP 2005-33665 A
In the network system as shown in FIG. 12, if an inter-device interface connecting the network relay device 100 and the network relay device 200 is formed of a low capacity line, frames might be discarded. In order to prevent occurrence of such a situation, network relay devices, arranged opposite each other, may be connected in multiple stages as shown in FIG. 13.
In the network device shown in FIG. 13, network relay devices 300 and 400, arranged opposite each other in the first stage, respectively transmit part of frames input from L2SW 303 and 403 to the counterpart devices via an inter-device interface 500, and transmit the rest of the frames to network relay devices 301 and 401 in the second stage via lines 304 and 404. The network relay devices 301 and 401 of the second stage respectively transmit part of the frames transmitted via the lines 304 and 404 to the counterpart devices via an inter-device interface 501, and transmit the rest of the frames to network relay devices 302 and 402 in the third stage via lines 305 and 405. The network relay devices 302 and 402 in the third stage respectively transmit the frames transmitted via the lines 305 and 405 to the counterpart devices via an inter-device interface 502. In this way, by connecting the network relay devices in multiple stages to realize redundant inter-device interfaces, as the load can be distributed to the inter-device interfaces, it is possible to reduce a risk of frames being discarded due to capacity shortage of the inter-device interfaces.
Meanwhile, even in the system as shown in FIG. 13, a deadlock may occur when link loss forwarding is performed. For example, when a link disconnection “c” occurs in the line 405 of the network system shown in FIG. 13, the network relay devices 401 and 402 respectively notify the network relay devices 301 and 302, which are the counterpart devices thereof, of occurrence of the link disconnection “c”. Upon receiving the notification, the network relay devices 301 and 302 respectively cause a link disconnection “d” forcibly, and notify the counterpart devices 401 and 402 of occurrence of the link disconnection “d”. Accordingly, the network relay devices 401 and 402 respectively recognize occurrence of the link disconnection “d”. As such, even when the link disconnection “c”, which is the original failure, is recovered, as the link disconnection “c” is forcibly caused, a deadlock state occurs. It should be noted that as the technique described in Patent Document 1 is for preventing occurrence of a deadlock which may be caused by link loss forwarding performed on a network system without any branching connections, such a technique is not applicable to a network system in which network devices such as network relay devices are connected in multiple stages to realize redundant inter-device interfaces.